This invention relates to thermostats and more particularly relates to thermostats employing anticipator resistors.
An elementary thermostat for a heating system employs a thermostatic switch which closes a circuit supplying power to a heat source when the air in a room controlled by the heating system cools below a predetermined temperature. The thermostatic switch shuts off the power to the heat source when the system has heated the air in the room to a second predetermined temperature. However, conventional heating systems, such as forced air gas furnaces, continue to supply heat to the room after the power to the heat source is shut off. Heat continues to be supplied because the heat exchanger and plenum of a conventional gas furnace acquire a considerable amount of heat from the heat source which would be wasted if the heat were not circulated to the room after the heat source is turned off. The continued supply of heat to the room results in an overshoot of the desired temperature which is not anticipated or corrected by an elementary type of thermostatic control.
To correct the overshoot of temperature inherent in the elementary control system described above, it is conventional to use a thermostat employing an "anticipator." The anticipator is a small resistance heating element which controls the cycle period of the heating system by supplying heat directly to the thermostat bimetal when the thermostatic switch contacts are closed, so that the contacts are opened before the surrounding environment actually reaches the desired temperature, thus "anticipating" the reaching of the desired temperature which occurs at a later time due to the continued transfer of heat from the heat source after the contacts are opened. The thermostat bimetal is thus responsive to both room temperature and heat from the anticipator. It is desired to maintain as closely as possible a control temperature and variations from such control temperature are called "droop."
The cycle rate is a function of the anticipator and it is preferred that the burner (in a gas furnace) remain on for a predetermined minimum period of time (for example, two and one-half to three minutes) in order to adequately heat the heat exchanger surface and prevent cold spots from being below the dew point of the flue products, which cold spots might result in condensation that could cause corrosion and decrease the life of the heat exchanger. The minimum cycle period is desirably just long enough to evaporate condensed combustion products of the heating system and is somewhat variable depending on the burner parameters. A cycle includes both burner on time and burner off time, for example, six cycles per hour might include six 3 minute on cycles and six 7 minute off cycles.
the anticipator in conventional thermostats is adjustable to compensate for certain operating conditions, as for example, chronic low voltage. However, such adjustment might undesirably alter the cycle rate.
Manufacturing variations cause wide variance in thermostat performance, and only increase problems in control caused by variances in load and current in use. A faulty voltage supply to a conventional anticipator can cause variable cycle periods. In most systems, the anticipator is powered by a varying unregulated voltage supply in which the voltage varies with changes in line voltage and amperage load on the transformer. The instability of the voltage drastically affects the cycle period resulting from the operation of the anticipator.
If corrosion were the only problem, anticipators could be adjusted to provide extra long cycle periods. Unfortunately, this "solution" would result in widely fluctuating room temperatures meaning considerably less comfort and increased fuel consumption.
Erratic cycle periods due to faulty voltage supplies might be better controlled by regulating the voltage supplies. However, this is an expensive undertaking which also increases the maintenance costs of the resulting heating and/or cooling system.
It has been discovered that the deficiencies of conventional anticipators can be overcome and improved operation can be achieved by providing an anticipator which dissipates heat at a substantially constant rate over a wide variation of supply voltages and current. Such an anticipator enables the on cycle period to be maintained above a minimum time duration to provide longer life for the heat exchanger in the furnace and increase the comfort level.
the applicant has also discovered that heating system cycle periods can be stabilized without resorting to costly regulated voltage supplies by employing an anticipator which dissipates heat at a substantially constant rate over a range of supply voltages.
According to another aspect of the invention, the applicant has discovered that improved anticipator operation can be achieved if the anticipator comprises a positive temperature coefficient (PTC) thermistor.
According to another feature of the invention, an anticipator capable of dissipating heat at a substantially constant rate can be used in conjunction with a cooling system in order to afford advantages similar to those discussed above in connection with heating systems. According to this feature of the invention, the cooling anticipator is also fabricated from a thermistor having a positive temperature coefficient.